Active airborne electromagnetic (AEM) systems are used to map and detect geological features in the ground according to their varied electrical conductivity. Many metal ore deposits are more electrically conductive than their host rocks, and AEM surveys have been successful in many regions of the world in helping discover new ore resources. An AEM system may include equipment that is carried on or towed by an aircraft. The aircraft and AEM equipment are flown over the ground to be investigated.
An active AEM system generally comprises a transmitter and a receiver. The transmitter creates a primary field to induce eddy currents the Earth which may be detected with the receiver. The transmitter usually comprises a loop of one or more electrically conductive turns through which an electrical current is driven to create the primary magnetic field. The electrical current of the loop, which is time-varying and is usually given a periodic waveform, is supplied by a transmitter driver module that is energized by an electrical power source on or carried by the aircraft. The resulting primary magnetic field surrounds and extends away from the transmitter loop; its intensity diminishing rapidly with distance from the loop. The primary magnetic field cuts through conductive ground and induces eddy currents in the ground. The eddy currents generate their own secondary magnetic field, which may be detected using the receiver sensor of the AEM system. The receiver sensor may comprise a magnetometer, or any sensor for detecting a magnetic field or its change. The received signals provide information about the geometrical distribution and extent of electrical conduction in the earth under the AEM system.
The receiver is usually located close to the transmitter, at a point where the primary magnetic field is enormously stronger than typical secondary fields. Therefore, an AEM system must generally provide means for the receiver to detect and separate whatever weak secondary field reaches the receiver location from the primary field.
There are at least two approaches for performing the primary-secondary separation. One approach involves attempting to annul the primary field of transmitter at the location of the receiver sensor by bucking or angular orientation. A second approach involves limiting the secondary measurements to an “off-time” interval in the primary current waveform, or by its frequency domain alternative of measuring only secondary components that are in quadrature phase with the primary field signal. Either method or variations of them may be employed, as alternatives or in combination.
The maximum depth at which a deposit of a given size, conductivity and geometry may be detected depends on the strength of the transmitted signal. This is proportional to the dipole moment of the system, which is the product of the transmitter current, number of turns and area of the transmitter loop. The detection of deep deposits may be enhanced by using a larger transmitter dipole moment.
The frequency of the transmitted signal may be another consideration. AEM systems generally may need to use a lower (base) frequency to penetrate deeper into conductive ground. Production of a strong transmitter signal at low frequency generally requires a transmitter loop that is physically a large part of the whole flight system.
Receiver motion noise is usually a significant consideration in increasing the sensitivity of an AEM system for low frequencies (e.g. below 20 Hz). Approaches to reduce noise at the receiver include vibration isolation systems for receiver sensors, and increasing of the signal by increasing the dipole moment of the loop by some combination of increasing its size, number of turns, peak current, and duty cycle. However, increasing the dipole moment of the loop may compromise the other desirable characteristics of an AEM system by affecting one or more of: the geometric stability of the receiver sensors, the dynamic range of the receiver, the primary-secondary field separation, the stability of flight, the ability to safely land and take off with the transmitter loop, or the ability of the loop to be transported from site to site.
Since AEM systems induce currents to flow in the ground by electromagnetic induction as described by Faraday's Law, in general as frequencies are lowered, the secondary fields will become weaker relative to the primary field. Thus, when a large primary field is present relative to the scattered field, such as is the case when low frequencies are employed, it may be advantageous to annul the primary field with one or more auxiliary coils. Such bucking (or “annulment”) has the advantage of permitting the sensor to be operated with a greater sensitivity than would otherwise be possible. Bucking may further enable better detection of weak fields by diminishing any stray currents induced in the region of the receiver which may be a source of noise. When bucking is used to boost the sensitivity to weak fields, rigid geometries between the bucking coils, the receiver sensors and the transmitter loop are generally preferred.
However, an increase in the size of a transmitter loop for producing strong transmitter signals at low frequencies generally comes with an increase in weight. An increase in the size or weight of a transmitter loop may pose some challenges in an AEM system, such as limiting the types of aircraft that may be used, limiting the maximum flight velocity of the system, and increasing the total aerodynamic drag on the system in-flight.